// Copyright (c) 2012 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef GPU_COMMAND_BUFFER_COMMON_COMMAND_BUFFER_SHARED_H_
#define GPU_COMMAND_BUFFER_COMMON_COMMAND_BUFFER_SHARED_H_

#include "base/atomicops.h"
#include "command_buffer.h"

namespace gpu {

// This is a standard 4-slot asynchronous communication mechanism, used to
// ensure that the reader gets a consistent copy of what the writer wrote.
template <typename T>
class SharedState {
    T states_[2][2];
    base::subtle::Atomic32 reading_;
    base::subtle::Atomic32 latest_;
    base::subtle::Atomic32 slots_[2];

public:
    void Initialize()
    {
        for (int i = 0; i < 2; ++i) {
            for (int j = 0; j < 2; ++j) {
                states_[i][j] = T();
            }
        }
        base::subtle::NoBarrier_Store(&reading_, 0);
        base::subtle::NoBarrier_Store(&latest_, 0);
        base::subtle::NoBarrier_Store(&slots_[0], 0);
        base::subtle::Release_Store(&slots_[1], 0);
        base::subtle::MemoryBarrier();
    }

    void Write(const T& state)
    {
        int towrite = !base::subtle::Acquire_Load(&reading_);
        int index = !base::subtle::Acquire_Load(&slots_[towrite]);
        states_[towrite][index] = state;
        base::subtle::Release_Store(&slots_[towrite], index);
        base::subtle::Release_Store(&latest_, towrite);
        base::subtle::MemoryBarrier();
    }

    // Attempt to update the state, updating only if the generation counter is
    // newer.
    void Read(T* state)
    {
        base::subtle::MemoryBarrier();
        int toread = !!base::subtle::Acquire_Load(&latest_);
        base::subtle::Release_Store(&reading_, toread);
        base::subtle::MemoryBarrier();
        int index = !!base::subtle::Acquire_Load(&slots_[toread]);
        if (states_[toread][index].generation - state->generation < 0x80000000U)
            *state = states_[toread][index];
    }
};

typedef SharedState<CommandBuffer::State> CommandBufferSharedState;

} // namespace gpu

#endif // GPU_COMMAND_BUFFER_COMMON_COMMAND_BUFFER_SHARED_H_
